CCIE Pursuit Blog

April 5, 2008

Internetwork Expert Volume II: Lab 6 – Section 4

Exterior Gateway Routing – 11 Points

4.1 BGP Peering

Easy task as there are not a lot of devices running BGP and all but one peering is on r6.

“All BGP traffic between r4 and r6 should traverse the VPN tunnel.”

Easy enough,  just make the neighbor addresses 191.1.46.x.  This also save me from configuring ‘ebgp multihop’ for the r4 <-> r6 peering.

4.2  BGP Bestpath Selection

Send all traffic for prefixes learned from AS 54 on r6 to BB1.  Don’t use local_pref.

What about weight?

Best Path Selection Table:

Attribute Direction Applied Traffic Flow Affected Prefer
Weight Inbound Outbound High
Local_Pref Inbound Outbound High
AS-Path Outbound Inbound Shortest
MED Outbound Inbound Lowest

Just set the weight from routes learned from BB1 to something greater than the default of 0 (IE used 100, I used 65000):

r6(config-router)#do sh ip bgp | i
*>               0         65000 54 50 60 i
*>               0         65000 54 50 60 i
*>               0         65000 54 i
*>               0         65000 54 i
*>               0         65000 54 i
*>               0         65000 54 i
*>               0         65000 54 i
*>               0         65000 54 i

4.3 BGP Filtering

Configure r6 so that AS 100 will not accept any prefixes from as 54 with a mask longer than /20.  Use a single-line prefix-list.

The prefix-list bit is easy:

r6(config)#ip prefix-list LESSTHANTWENTY perm le 20

There’s a really nice breakdown on prefix-lists in general in the IE solution guide.

I was thrown off by the SET_WEIGHT route-map in the IE solution guide, but it’s same route-map name they used in the previous task (I had named mine ‘WEIGHT’) with an additional line.  So your final route-map should look similar to this:

route-map WEIGHT permit 10
 match ip address prefix-list LESSTHANTWENTY
 set weight 65000

task 4.3

4.4 BGP Summarization

Configure a summary of and but do not use aggregate-address.  You are allowed to use two static routes on r3 to accomplish this.

I had NO CLUE on this one.  I peeked the solution guide and then slapped my big, dumb forehead.  The ip routes are to NULL0.  You then redistribute the static routes.  DOH!!!

r3(config)#ip route null0
r3(config)#ip route null0
r3(config)#router bgp 200
r3(config-router)#redist static

r6#sh ip route bgp | i 150|191.1. is variably subnetted, 21 subnets, 4 masks
B [20/0] via, 00:01:00      is variably subnetted, 10 subnets, 3 masks
B [20/0] via, 00:01:00 

4.5 BGP Table Stability

Configure r6 to not advertise and if they are “consistently unstable.”

bgp dampening

Usage Guidelines
The bgp dampening command is used to enable BGP route dampening. This command can be entered without any arguments or keywords. The half-life, reuse, suppress, and max-suppress-time arguments are position-dependent; meaning that if any of these arguments are entered, then all optional arguments must be entered.

When BGP dampening is configured and a prefix is withdrawn, BGP considers the withdrawn prefix as a flap and increases the penalty by a 1000. If BGP receives an attribute change, BGP increases the penalty by 500. If then the prefix has been withdrawn, BGP keeps the prefix in the BGP table as a history entry. If the prefix has not been withdrawn by the neighbor and BGP is not using this prefix, the prefix is marked as dampened. Dampened prefixes are not used in the BGP decision process and not installed to the routing table.

router bgp 100
 bgp dampening route-map FLAPPERS
ip prefix-list FLAPPERS seq 5 permit
ip prefix-list FLAPPERS seq 10 permit
route-map FLAPPERS permit 10
 match ip address prefix-list FLAPPERS

I missed on important piece though:

*Mar  6 18:15:27.386: %BGP-3-BADROUTEMAP: Bad parameters in the route-map FLAPPERS applied for Dampening

You MUST specify dampening parameters in the route map.  Just use the defaults:

BGP dampening is disabled by default. The following values are used when this command is enabled without configuring any optional arguments:

half-life: 15 minutes
reuse: 750
suppress: 2000
max-suppress-time: 4 times half-life

r6(config-router)#route-map FLAPPERS perm 10
r6(config-route-map)#set dampening 15 750 2000 60

r6#sh ip bgp dampening parameters
 dampening 15 750 2000 60 (route-map FLAPPERS 10)
  Half-life time      : 15 mins       Decay Time       : 2320 secs
  Max suppress penalty: 12000         Max suppress time: 60 mins
  Suppress penalty    :  2000         Reuse penalty    : 750



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